1. Field of the Invention
The present invention relates to a magnetic tape where a servo signal to perform tracking control for a magnetic head is written, a manufacturing method thereof, and a servo writer.
2. Description of the Related Art
These years, in a magnetic tape a high density recording has progressed and there is the tape having a capacity of around 100 gigabytes for a backup of a computer. Therefore, several hundreds of data tracks are formed in the magnetic tape in a lateral direction. Accordingly, a width of a data track extremely becomes narrow, and also a distance between adjacent data tracks extremely becomes narrow. Therefore, in order to make a recording/reproducing element of a magnetic head trace the data track, a servo signal is written in the magnetic tape in advance and with reading the servo signal by the magnetic head, a position of the magnetic head (position of the lateral direction of the magnetic tape) is servo-controlled (see Japanese Patent Laid-Open Publication No. Hei 8-30942).
And the servo signal is recorded by giving a recording current to a servo band on non magnetized magnetic tape so as to magnetize the servo band in one direction.
In other words, as shown in FIG. 8B, conventional servo signals SS are formed on non magnetized servo bands SB by flowing a recording current pulse PC consisting of a zero current and a plus pulse current as a recording current in order to avoid a saturation phenomenon of an MR (Magneto Resistive) element. If such the recording current pulse PC is used, as shown in FIG. 8B a magnetic tape MT is not recorded in areas except for servo patterns SP when the recording current pulse PC is the zero current; and when the plus pulse current of the recording current pulse PC flows, the servo patterns SP are magnetized in one direction by a leak magnetic flux from a servo gap, thereby as a result the servo signals SS being written. On the other hand, in a magnetic tape recoding/reproducing apparatus a change point of a magnetization in the servo signals SS is detected with a change of an electric resistance by a servo signal reading element (MR element), and the change point of the magnetization as a read signal is output in a differential waveform (voltage value). Therefore, the larger the electric resistance of the MR element becomes, the higher a peak value of the read signal of the servo signals SS, thereby an SN (Signal/Noise) ratio of the read signal being improved. Accordingly, when changes of the servo signals SS themselves are large and when a read area is large due to a wide width of the servo signal reading element (MR element), as shown in FIG. 8C a peak voltage value of a read signal RSL of the servo signals SS becomes high.
Whereas, hereafter the high density recording of the magnetic tape is foreseen to progress till around several tens of terabytes. Therefore, a number of data tracks of the magnetic tape, the width of the data track, and the distance between the adjacent data tracks become narrower, and the magnetic tape-itself becomes a thin layer. Based upon this, an amount of magnetism that is detectable in reading a servo signal decreases and a change of a magnetization amount that is detectable with the servo signal reading element also becomes small. Accordingly, as shown in FIG. 8D a peak voltage value of a read signal RSS of the servo signals SS becomes small, thereby the SN ratio of the read signal RSS worsening. As a result, in the magnetic tape recording/reproducing apparatus the servo signals SS becomes not able to be accurately read, thereby highly accurate position control of the magnetic head being not able to be performed.
Consequently, an improvement of the SN ratio of a read signal of a servo signal and a manufacturing method thereof are requested in the magnetic tape.